1. Field of the Invention
The present invention relates to a semiconductor wafer and a method of heat-treating the same and, more particularly, to a heat-treating method in a donor killing process which is performed to remove a role of donor of interstitial oxygen in the crystal during single-crystal growth in a semiconductor wafer fabricating process and a semiconductor wafer shaped by the heat-treating method.
2. Discussion of Related Art
Very large scale integration circuit (VLSI) is that high-density devices are integrated on a single-crystal semiconductor wafer. A general semiconductor wafer fabricating process will be described with reference to FIG. 1.
First, a single crystal is grown from raw materials. The single-crystal growing process is that after the raw materials like quartzite, etc. are shaped into a polycrystalline silicon through complex and multilevel purifying process, it is grown into a single-crystal ingot by the Czochralski (CZ) technique or the Float Zone (FZ) technique.
Following the growth of the single-crystal ingot, a wafer, being an adequate material in semiconductor device fabrication, is shaped through performing a series of complex shaping and polishing process. This process is called "watering".
That is, since the surface of the single-crystal ingot is ruggedly formed, a trimming process is necessary for trimming the ingot surface to have an adequate shape and size. The ingot trimmed is oriented in the desired direction along its length after one or more flats have been examined by X-ray (Orientation Flattening). Then, an ingot etching operation is performed to remove contaminants from the ingot surface and a slicing process is performed to make a silicon slice out of the ingot. The slicing process is carefully performed in order to correctly keep the crystal direction of the slice. Thereafter, the edge of each silicon slice, sliced into that having a predetermined thickness, is rounded to give a continual convenience in treating the wafer and not to accumulate a layer on the edge of the wafer in the subsequent device fabricating (Edge Rounding).
Thereafter, the slice is lapped by the use of mixtures of oxide aluminum and glycerin to prevent it from bending so that its flatness increases. The previously described shaping operations leave the surface and edges of the wafer damaged and contaminated. The damaged and contaminated regions can be removed by chemical etching (Slice Etching).
Following that operation, a heat-treating process called "donor killing" is performed. The donor killing process is generally performed in a thermal furnace at 600 to 650.degree. C. for 30 minutes. While, in a Rapid thermal Annealing (RTA) device, the process is performed around 700.degree. C. for 30 seconds. Since the interstitial oxygen existed in the single-crystal lattices of the silicon is positively or negatively charged to thereby have a role of donor in the crystal lattices, the donor killing operation is to remove the interstitial oxygen which deteriorates the electrical control by use of an implantation process with respect to the wafer. The phenomenon caused by the interstitial oxygen is eradicated by performing a heat-treating operation above 500.degree. C.
Then, the surface of the heat-treated slice is polished chemically or mechanically and cleaned. An inspection in the defects and orientation of the cleaned wafer is performed and an packing is performed for the passed wafer.
Meanwhile, when a single-crystal silicon is grown by the conventional Czochralski technique, plentiful of oxygen is generated. It typically exists in the range of 5.times.10.sup.17 to 1.times.10.sup.18 atoms/cm.sup.3 (or 10 to 20 ppma). The initial oxygen (Oi) induced in the crystal growing process is atomically dissolved and occupies the interstitial sites of the lattice. It becomes the most important precipitation material in the silicon wafer shaped by the Czochralski technique due to its characteristics that the degree of diffusion is very high and the solubility rapidly falls at low temperature. The oxygen precipitates grown from the initial oxygen is extremely undesirable in the electrical characteristic of the semiconductor device and is closely connected with the Electrical Die Sorting (EDS) process or the yield of the package. Thus, it should be removed or suppressed.
FIG. 2 is a graph measuring the correlation between the initial oxygen concentration and the oxygen precipitate density after performing the Dynamic Random Access Memory (DRAM) process on the particular wafers passing through the conventional donor killing process.
The wafers #1 to #6 are silicon wafers fabricated through the single-crystal growing process by the Czochralski technique, and are products of particular wafer manufacturers. Each wafer previously passed the gate oxide module formation during the DRAM process. Concerning each wafer, the density of the oxygen precipitates existed in the active region of DRAM is measured by means of the Laser Scattering Tomography (LST) device. Distribution of the oxygen precipitates measured lies in the range of 200 to 400 .mu.m.
From FIG. 2, it can be known that the density of the oxygen precipitates in each wafer increases as does the initial oxygen concentration. That is, the initial interstitial oxygen induced into the crystal during single-crystal growth acts as the nucleation-element of the oxygen precipitates.
FIG. 3 is a graph measuring the correlation between the initial oxygen concentration and the substrate leakage current after performing the semiconductor DRAM process concerning each wafer processed from the given ingots.
These measuring operations are performed with respect to the ingots #1 to #6 in the substrate voltage 20 V. From FIG. 3, it might be expected that the substrate leakage current usually increases with respect to the ingot having higher initial oxygen concentration. The reason is that the oxygen precipitates increase in accordance with the increase of the initial oxygen concentration so that the leakage element of the semiconductor substrate increases.
FIG. 4 is a graph measuring the correlation between the initial oxygen concentration and the Bin 1 yield with respect to the wafers passed the semiconductor 16 M DRAM.
The Bin 1 yield indicates that passed the Bin 1 test (so-called, prime good) which is done as a step of the Electrical Die Sorting (EDS) process performing an electrical characteristic test of each chip with respect to the wafer embodying particular devices.
FIG. 4 shows that the Bin 1 yield decreases in the relation of the complementary error function, indicated by a solid line, as the initial oxygen concentration increases. That is, rapid decrease in the Bin 1 yield occurs when the initial oxygen concentration is above 12.50 ppma (parts per million atoms).
FIG. 5 is a graph representing difference in oxygen concentration (.DELTA.Oi) before and after the heat-treatment versus the initial oxygen concentration for obtaining the good yield in the semiconductor DRAM fabricated by the use of the usual semiconductor wafer. The oxygen concentration difference is measured with the Fourier Transform Infrared (FTIR) spectrometer. As shown in the drawing, the wafer is heat-treated in a thermal furnace at 700.degree. C. for 20 hours, and in succession, at 1000.degree. C. for 10 hours.
As might be expected from the correlation between FIGS. 4 and 5, the Bin 1 yield rapidly decreases around the initial oxygen concentration 12.50 ppma, being an alteration point. And correspondingly, when the initial oxygen concentration is above 12.50 ppma, the oxygen concentration difference (.DELTA.Oi) rapidly increases. On the contrary, when the initial oxygen concentration is below 12.50 ppma, the Bin 1 yield is kept very high above 35 % and the oxygen concentration difference is kept very stably below 2 ppma.
Therefore, it is required in the semiconductor wafer fabrication that the initial oxygen concentration in the semiconductor wafer is kept below 12.50 ppma to obtain the good yield of the device.
However, to keep the initial oxygen concentration in the semiconductor wafer below an permissible value, for example, 12.50 ppma, not only a careful interest should be made from the beginning of single-crystal growth, but also a high cost of single-crystal growing device is required. Thus, the cost of the semiconductor wafer increases.
Also, even in a high precision of single-crystal growing device, it is very difficult to precisely control the concentration of the initial oxygen induced during crystal growth.
Furthermore, although the initial oxygen concentration in the semiconductor wafer might be precisely controlled, distribution of the initial oxygen concentration with respect to each wafer is variously dispersed so that it is difficult to precisely control the characteristics of the wafer.